The present invention relates generally to gas turbine engines, and, more specifically, to combustors thereof.
A typical gas turbine engine combustor includes radially outer and inner annular combustion liners joined at forward ends to an annular dome. The combustor dome includes a plurality of carburetors for injecting a fuel and air mixture into the combustor which is burned for generating hot combustion gases. Incomplete combustion results in undesirable exhaust emissions including unburned hydrocarbons and nitrogen oxides (NOx).
Accordingly, gas turbine combustor art is quite crowded with various configurations of the basic combustor and individual carburetors for maximizing engine performance while reducing undesirable exhaust emissions with varying effectiveness.
A typical carburetor includes an air swirler which includes one or more rows of air swirling apertures or vanes that swirl compressor discharge air around fuel being injected centrally therein. Typical fuel injectors are configured to cooperate with the swirlers for finely atomizing the fuel to produce a suitable fuel and air mixture for undergoing combustion.
However, the available axial space or envelope in the combustor is a design constraint for maintaining the size, weight, and cost of the overall engine within limits, especially for an aircraft engine application where size and weight are critical design objectives. Accordingly, the ability to completely mix the injected fuel with the air prior to combustion is constrained by the available axial length.
Government mandated environmental regulations are increasingly limiting the permitted levels of undesirable exhaust emissions, including in particular the NOx levels. However, this is quite difficult since NOx and hydrocarbons emissions during the combustion process occur differently at different temperatures and are subject to the available residence time of the combustion gases inside the combustor.
A recent development program for a high speed civil transport (HSCT) aircraft engine includes a new type of combustor based on premixing of the fuel and air in an enclosed passageway prior to combustion for further reducing exhaust emissions, and in particular the NOx levels. Two recent U.S. patent applications for this new combustor are Ser. Nos. 09/398,557 and 09/398,559 filed concurrently on Sep. 17, 1999.
These types of combustors are configured as lean premix pressurized (LPP) combustors which include elongate integrated mixer flame holder (IMFH) mixer tubes that receive fuel at upstream inlet ends thereof which is mixed with air inside the respective tubes for being premixed over a substantial axial length prior to discharge into the combustion chamber. And, a row of pilot fuel injectors surrounds the combustor for cooperating with the primary fuel injectors and tubes to provide suitable combustor performance from idle to maximum power, while substantially reducing NOx emissions for the HSCT aircraft engine application.
However, since the mixer tubes are relatively long and are supported at opposite ends thereof, they are subject to substantial differences in external temperature during operation. The tube outlet ends are suspended from a dome casing, and are directly subject to the hot combustion gases produced downstream therefrom. And, the inlet ends of the tubes are supported in a relatively cold forward support plate.
In the development combustor, an array of the mixer tubes is brazed to a common heat shield segment. And, due to the substantial differential temperatures in the combustor dome, the tube brazes are subject to limited fatigue life as the tubes tilt or cock under differential expansion and contraction. Replacement of an individual mixer tube requires disassembly of the entire heat shield segment with its corresponding array of mixer tubes, with subsequent repair or replacement of the individual tubes. This is a relatively complex maintenance operation, and is expensive.
Accordingly, it is desired to provide an improved LPP combustor having a new configuration of the mixer tubes for improving combustor life and reducing maintenance complexity and costs.